Controlling a single molecules reaction with light could improve solar cells

In the race to make solar energy cheaper and more efficient, many groups are synthesizing or tweaking the light-capturing molecules. A new study goes one step further: it uses nanotechnology tools to steer individual molecules to react in new ways under the influence of light.

The research, by researchers at the University of California, Los Angeles and the University of Washington, was published last week in the journal Science.

“This Science paper is just the tip of the iceberg,” said co-author Alex Jen, the Boeing-Johnson Chair professor of materials science and engineering and professor of chemistry at UW. “This work will allow scientists to synthesize much better-designed molecules to study photovoltaic properties: How do we harvest light and generate current at the molecular level? If you understand the mechanism for this light-to-electrons process, then you can improve the efficiency of solar cells.”

The researchers are interested in organic molecules that can convert solar energy to electricity. These are less efficient than todays commercially available silicon-based solar cells, but if scientists can improve their performance they promise to deliver cheaper, thinner, lighter and more flexible solar technology.

An illustration of how nanotechnology can steer molecules to react in a particular way.UCLA

“This is one step in measuring and understanding the interactions between light and molecules, which we hope will eventually lead to more efficient conversion of sunlight to electrical and other usable forms of energy,” said lead author Paul Weiss, a UCLA professor of chemistry, biochemistry and materials science & engineering who holds the Kavli Chair in Nanosystems Sciences.

Results could be applied to developing organic light-emitting diodes, used in flat-screen TVs and digital displays. The overall technique could also be used to study other types of molecular reactions.

The UW team designed the molecules used in the study. In a 2003 study published in the journal Nano Letters, Jen and colleagues showed that these molecules use a sulfur atom as an anchor and attach to a smooth gold surface in a tightly packed, controlled array.

In the study published in Science, first author Moonhee Kim, a Pennsylvania State University doctoral student carrying out her research at UCLA, used a concept similar to one seen in a toddlers toy that has cutouts that accept only certain shapes. Kim created defects, or cutouts, in the gold sheet that would only accept two of the molecules, forcing them to align in a predetermined way. When illuminated with light, these two molecules – rooted in place – react predictably in a way seldom seen in nature because it requires too much energy.

Until now, the way reactions occur in solution has been something of a black box. Researchers have little control over the reaction and cannot directly observe whats happening.

Fixing the molecules to the gold surface forces them to align in a certain way, and this favors reactions that would not happen otherwise. In nature, enzymes use this approach to promote particular biochemical reactions.

The UCLA researchers used a custom high-resolution microscope to watch the two molecules interact under ultraviolet light. Researchers were able to see the molecules attach to the gold surface, and saw what happened when UV light shone on the molecules, and after the light stopped shining.

In the future, scientists could use this technique to design and test new solar molecules.

“How can you know what happens, on the molecular scale, when you shine light on the molecule?” said co-author Hong Ma, a UW research assistant professor of materials science and engineering. “You have all kinds of phenomena, charge separation, charge generation and charge transport. This paper lays the foundation for understanding these photon-induced phenomena by monitoring them on the molecular scale. Then you can design a better material for the solar cell.”

The team is now doing follow-up work using a similar technique with newer molecules, also designed at the UW, that not only respond to light but also can convert light to electricity.